In recent years, point light sources such as light emitting diodes (LED) and laser diodes have been used as light sources for various display devices, illumination devices, and others in place of incandescent bulbs and fluorescent tubes because such point light sources consume less electricity, generate less heat, and have longer service life.
There are various illumination devices using a point light source. Examples of such illumination devices include square and round illumination devices that are attached to a ceiling or a wall in a building to be used for, for example, room illumination, a miniature bulb-type illumination device used as, for example, an outdoor lamp, and a linear illumination device similar to a fluorescent lamp. Recently, LED illumination devices having an incandescent bulb shape, a linear fluorescent lamp shape, and an annular fluorescent lamp shape have been developed so that such an LED illumination device can be used in place of related art incandescent bulbs or fluorescent lamps.
A surface illumination device that makes light emitted from a point light source uniform on a predetermined surface is classified into a so-called backlight-type device in which a light source is disposed behind an illumination surface to face to the illumination surface and a so-called edge light-type device in which an illumination surface is orthogonal to a light source. The edge light-type surface illumination device typically employs a light guide plate and can be made thinner. The light guide plate is, however, required to be provided all over an illumination surface, thereby increasing the weight and the production cost. In contrast, some backlight-type surface illumination devices do not need the light guide plate. Such a case enables weight reduction. The backlight-type surface illumination device is therefore used at a place where weight reduction is required. LEDs and laser diodes are, however, point light sources having strong directivity, thereby generating strong light in an optical axis direction, namely, so-called glare. Thus, in order to provide uniform light distribution over a wide surface as an illumination device, various improvements are required.
For example, as an LED illumination device having a fluorescent lamp shape using a point light source, Patent Document 1 (JP-A-2008-77899) discloses a technique for an LED lamp that includes a reflection plate in a linear or annular outer shell case having a U shape in section and having an opening on one face, an LED attached to a bottom of the reflection plate, and a diffusion sheet for diffusing light provided near the opening of the outer shell case.
FIG. 19 is a sectional view of the LED lamp described in Patent Document 1. In the LED lamp 101, a reflection plate 103 is disposed on an opening side of an elongated outer shell case 102 having a U shape in section so that light is emitted toward the opening. In the reflection plate 103, a plurality of LED modules 105 in which an LED 105a is attached to a substrate 105b are linearly disposed. Around the LED module 105, a reflector 105c made of resin or metal optimally designed so as to efficiently reflect light in the opening direction is provided. A diffusion sheet 104 for diffusing light is attached to the opening of the outer shell case 102 to convert LED light into a linear light source or an omnidirectional light source. This invention can provide an LED lamp in which the number of LEDs to be used is reduced, and that can be used in place of a linear or annular fluorescent lamp.
Patent Document 2 (JP-A-2005-251660) discloses, as shown in FIG. 20, an illumination device 201 in which annular light sources are concentrically disposed. The illumination device 201 includes a metal substrate 209 having an annular outer shape and having an open space part 211 for disposing a luminous body 210, annular light sources 202 to 204 including LEDs 219 and luminous bodies 210 that are arranged in the open space part 211 and are in contact with the metal substrate 209, and a metal main body 205 that has a bottom face in contact with back faces of the annular light sources and in which the annular light sources are arranged so that an inner peripheral face of one of the annular light sources is in contact with an outer peripheral face of another annular light source, or one of the annular light sources is in contact with a side face 229 or 230. The annular light sources 202 to 204 are provided with, in the open space part, a metal cover 206 that is opposed to the LED 219 and has holes 227 through which light emitted from the LED can be passed.